The process of manufacturing an optical fiber conventionally includes fabricating a preform and then drawing the preform into a fiber. The fiber-drawing operation, i.e. the transformation of the preform into a fiber, is conventionally effected by drawing without contact, which entails melting the end of the preform in an induction furnace filled with an inert gas. The diameter of the fiber is measured at the exit from the furnace and the measured diameter is used to control the drawing speed in order to maintain the diameter of the fiber constant. The fiber-drawing speed can be in excess of 15 meters per second (m/s).
During the fiber-drawing operation it is typical for the fiber downstream of the furnace outlet to be coated immediately with a primary coating, which is generally a resin that is polymerized by ultraviolet light. The coating serves in particular to protect the fiber against chemical attack and mechanical abrasion during subsequent handling, to absorb stresses, to prevent the propagation of cracks, and to absorb cladding modes by virtue of its slightly higher index. Epoxy acrylate or polyimide resins are typically used. Sometimes a more rigid secondary coating is applied to the fiber after applying the primary coating.
Optical fiber coating devices have been developed which prevent the optical fiber touching any solid surface before or during the coating operation, which is imperative.
Document U.S. Pat. No. 4,531,959 proposes a coating device of the above kind in the form of an injector for applying a coating to an optical fiber which limits the quantity of air bubbles trapped in the coating applied in this way. The injector includes a jacket of circular section having a stepped bore concentrically housing a guide die, a cylindrical grid, and an exit die. The guide die has a tapered axial orifice, reducing in size inwards from the outside toward the grid. The exit die has a tapered axial orifice, reducing in size outwards from the grid toward the outside. Between them, the jacket and the grid form a chamber which is connected to the periphery of the jacket by a plurality of radial passages. To form this chamber, the grid comprises a flange at each end to provide a locating surface within the jacket. The injector is placed in a support enabling it to be positioned relative to the optical fiber to be coated downstream of the fiber-drawing furnace. The support also feeds resin under pressure to the radial orifices of the jacket and thence into the chamber, where it passes through the grid. The optical fiber passes through the injector, which it enters via the orifice of the guide die, enters the grid, and leaves via the exit die. As it passes through the grid, the optical fiber is coated with resin to form a coating whose outside diameter is determined by the smallest diameter of the exit die.
The above injector cannot assure a sufficiently accurate concentric relationship between the resin coating applied to the fiber and the fiber itself because of the tolerance between the stepped bore of the jacket, on the one hand, and the dies and the grid on the other hand. Because of these tolerances, these components can take up a slightly skewed or eccentric position, instead of being aligned concentrically. Also, there is no seal between the stepped bore, on the one hand, and the grid and each of the dies, on the other hand, and the resin under pressure can therefore escape from the chamber to the outside by passing between the stepped bore and the locating flanges of the grid and then between the stepped bore and the dies. It is possible to fit seals, for example O-rings, between the stepped bore and the flanges of the grid and/or the dies. However, such seals worsen any defective positioning of the dies and/or the grid and the resulting defective concentric relationship between the coating and the fiber.